Lithography leap creates 20-nm chip features

MADISON, Wis.  Scientists at the University of Wisconsin have found a way to create 20-nanometer chip feature sizes with 100-nm masks, giving an unexpected leap to Moore's Law and possibly extending the life of current lithography.

The so-called "bright-peak technology" adjusts the space between a mask and a wafer to control the phases of X-ray lithography. "We learned how to use phase-shifting to control diffraction  a technique that works for X-rays or even traditional optical lithography," said professor Franco Cerrina, who created bright-peak enhanced X-ray phase-shifting masks with professor James Taylor and researcher Lei Yang at the Center for Nanotechnology here.

"With this bright-peak technology, you could write a 100-nanometer mask feature and wind up with a 20-nanometer chip feature," Taylor said. Such fine feature sizes are located at "about 2010" on the International Technology Roadmap for Semiconductors, he said.

The technology, developed in cooperation with the Synchrotron Radiation Center here, gives an eight-year leap to Moore's Law, the inventors said. Posited by Intel Corp. cofounder Gordon Moore, Moore's Law states that the number of transistors that can be placed on a chip will double every 18 months.

According to the University of Wisconsin inventors, today's chips that use 248-nm or 193-nm photomasks can only attain features as small as 100 nm in the photoresist layer of a chip. Common industry wisdom holds that chip makers will have to abandon lenses to focus light through masks onto a chip at dimensions below 100 nm, and will instead require special mirrors, since quartz lenses absorb too much light to go to sub-100 nm.

The Wisconsin inventors discovered the bright-peak mask technology while exploring the limits of X-ray lithography. They found that diffraction was the major factor preventing light from getting through the smaller features on a mask. Mask makers already tinker with masks to let more light through, but the inventors discovered they could control the phase of X-rays by adjusting the gap between a mask and wafer. The patent for their work is held by the Wisconsin Alumni Research Foundation.

Bright-peak masks work by positioning adjacent phase-shifting features so that light at the edges of small lines is bent by interference back toward the center of the line  the namesake for "bright peak" because the center of all the small lines become illuminated. As a result, chips features can be honed down sixfold compared to masks without the phase-shifting gaps.

Taylor is currently characterizing the bright-peak masks for optimal resolution, sensitivity and long-term stability as determined by manufacturing parameters such as thickness of mask material, phase angle, wavelength of exposing light, resist material, and size of desired features.